The invention relates to a device for detecting the simultaneous deviation of m of n alternating current (a.c.) signals from a predetermined set of signal characteristics. More particularly, the present invention relates to a monitoring circuit for generating a false condition output signal whenever m of n input signals deviate from a predetermined set of signal characteristics.
Customarily, in "failsafe" engineering a monitor network is impressed with an a.c. test signal, which appears at the network output only (if among other requirements), non element errors defined in any error catalog appear in the monitor circuit itself. Usually, to acquire and process monitored data, non-failsafe processing devices, e.g., computers also are used. In such cases, failsafety is insured by parallel-switching several of such non-failsafe devices and by monitoring their outputs with failsafe circuits.
For this purpose it is necessary to provide a monitoring circuit, which, being failsafe itself, generates a single output signal from all the output signals of the processing devices being monitored in accordance with the criterion selected. Since a.c. signals are usually used as test signals, such a monitoring circuit must generate an a.c. output signal as a function of the a.c. output signals generated by the devices being monitored.
The purpose of the present invention is to provide such a monitoring circuit. To meet above objects, the monitoring circuit of the present invention: (1) generates a direct current (d.c.) or "false" signal whenever at least m of the n a.c. input a.c.-signals simultaneously exhibit maxima and minima, which either both lie above or both lie below at least one preset signal range; and (2) generates an a.c. or "true" signal if the maxima and minima of at least m of n input signals respectively lie above and below said preset range. The monitoring circuit may also be designed to monitor additional characteristics of the input signals. For example, the monitoring circuit may be designed to generate a d.c. signal whenever m out of n input signals fall without a predetermined frequency range. As used herein, the term "preselected set of characteristics" is used to describe that set of characteristics (e.g., magnitude, frequency) of the input signals which are monitored by the monitoring circuit.